A sheet processing apparatus discriminates denominations, authenticities (authentic or counterfeit bills), and good/bad states (good bills as reusable banknotes or bad bills as non-reusable banknotes) in association with sheets. The sheet processing apparatus sorts sheets based on these discrimination results. It is not easy for a sheet processing apparatus of this type to set reference values (reference levels) required to discriminate sheets into good and bad bills according to their degrees of contamination or damage. This is because the required reference levels are different for users or the states of sheets to be processed are different depending on operation forms. For example, when a user requests (designates) a ratio between good and bad bills (good/bad bill ratio) as the processing result, the sheet processing apparatus has to set reference levels that can attain the good/bad bill ratio requested by the user. For example, Jpn. Pat. Appln. KOKAI Publication No. 2007-87219 describes a method of discriminating good or bad states of sheets based on the good/bad bill ratio.
However, the reference levels that can attain the desired good/bad bill ratio largely vary depending on states of sheets to be processed (i.e., operation forms). For this reason, it is difficult to uniformly provide, from the manufacturer side of the sheet processing apparatus, settings of reference values (reference values) that can attain a good/bad bill ratio requested by the user. Therefore, in the conventional sheet processing apparatus, an operator or service person sets reference levels to attain a desired good/bad bill ratio over a long period of time with reference to the actual processing results of a large number of sheets. Such operations require human skills and labor hours. That is, a conventional sheet processing system suffers a problem that it is not easy to set optimal reference levels required to determine good or bad states according to the actual operation form and a user's request.